Numerous methods are known for the execution of electrophotography. At a general level, using a photoconductive material an electrostatic latent image is formed on an electrostatic latent image-bearing member (also referred to as a “photosensitive member” below) by various means. Then, a visible image is made by developing this electrostatic latent image with toner; as necessary the toner image is transferred to a recording medium such as paper; and a copied article is obtained by fixing the toner image on the recording medium by, for example, the application of heat or pressure. For example, copiers and printers are image-forming apparatuses that use such an electrophotographic procedure.
Such copiers and printers are currently being used in quite diverse environments, e.g., low-temperature, low-humidity environments as well as high-temperature, high-humidity environments, and are thus required to output high-quality images without being influenced by the environment. In addition, examples of use outdoors have been increasing quite recently in combination with the downsizing and simplification of image-producing devices, and there is thus also demand for a stable image output regardless of the environment.
The charging state of a toner can be altered by the use environment, and, as one of the image defects produced as a result of this, a phenomenon known as “ghosting” occurs in which density irregularities appear in the image. A brief description of “ghosting” is provided in the following.
Development proceeds through the transfer of toner carried by the toner-carrying member to the electrostatic latent image. During this time, fresh toner is supplied to the regions where the toner on the surface of the toner-carrying member has been consumed (regions corresponding to image areas), while unconsumed toner remains present as such in regions where there has been no toner consumption (regions corresponding to nonimage areas). As a result, a difference in the amount of charging is produced between the freshly supplied toner (hereafter referred to as the supplied toner) and the toner that has remained present (hereafter referred to as the residual toner). Specifically, the freshly supplied toner has a relatively lower amount of charge and the toner that has remained present has a relatively higher amount of charge. Ghosting is produced due to this difference (refer to FIG. 1).
This difference in the amount of charging between the residual toner and the supplied toner is caused by the fact that the number of times the residual toner is subjected to charging grows to large values, in contrast to the fact that the supplied toner is subjected to charging, i.e., is passed through the contact region between the regulating blade and the toner-carrying member (referred to below as the contact region), a single time.
On the other hand, in a low-humidity environment, toner charging is not suppressed since there is little moisture in the air, and a state is assumed in which the charge on the toner is easily ramped up. Due to this, a state ends up being assumed in a low-humidity environment in which the residual toner carries a high amount of charge and the difference in the amount of charge between the supplied toner and residual toner then grows larger and ghosting is further worsened.
To date, the addition of an external additive, e.g., alumina or titania, has been pursued as a method for improving ghosting.
For example, according to Patent Literature 1, alumina is externally added in combination with strontium titanate or hydrophobic silica having a regulated BET specific surface area in order to improve the flowability of the toner and improve its aggregative property.
According to Patent Literature 2, large-diameter alumina fine particles are uniformly and tightly attached to the toner in order to improve the transportability at the toner-carrying member by reducing the amount of release external additive.
While a certain effect is obtained according to each of these patent literatures, these effects are inadequate in a low-humidity environment, which is an environment that facilitates the appearance of ghosting.
On the other hand, in order to solve problems caused by external additives, toners that focus in particular on external additive release have been disclosed (for example, Patent Literatures 3 and 4); however, these again cannot be regarded as adequate with regard to the charging performance of the toner.
Moreover, Patent Literature 5 teaches stabilization of the development·transfer steps by controlling the total coverage ratio of the toner base particles by the external additives, and a certain effect is in fact obtained by controlling the theoretical coverage ratio, provided by calculation, for a certain prescribed toner base particle. However, the actual state of binding by external additives may be substantially different from the value calculated assuming the toner to be a sphere, and such a theoretical coverage ratio does not correlate with the ghosting problem described above and improvement has been necessary.